Various chemical compounds, commonly referred to as dopants, are ultilized in semi-conductor device fabrication. In general, the liquid dopant source material is stored in a bubbler container having a gas inlet and outlet fittings at its top, and a carrier gas is passed in through the inlet and bubbles through the liquid in the bubbler container before passing to the bubbler outlet. The vapor pressure of the liquid dopant varies with temperature and it is necessary to maintain the liquid dopant in the bubbler at a substantially constant preset temperature to control dopant level in the semi-conductor processing furnace.
In a typical installation, the source bubblers are placed on shelves in a gas cabinet at one end of the furnace tubes of the semi-conductor processing furnace. The furnace tubes are heated to a very high temperature, commonly of the order of 1000.degree. C. and the radiant heat from the furnace tubes raises the temperature in the gas cabinet and aggravates the problem of maintaining the liquid dopant source material at a preset temperature. The shelves in the gas cabinet are commonly spaced apart a distance no more than about twelve inches and this limits the overall height of the temperature controller with the bubbler container installed. In addition, the shelves in the gas cabinet have to accommodate a number of other components including flow controllers and tubing which are connected to the ends of the furnace tubes. It is accordingly very desirable to make the temperature controller as small as possible to minimize the shelf space occupied by the temperature controller.
Thermoelectric temperature controllers have heretofore been made for controlling the temperature in the liquid chemical bubbler container. In general, such thermoelectric temperature controllers utilized semi-conductor type thermoelectric devices for either heating or cooling the bubbler container, to establish and maintain the liquid in the container at a preselected temperature. Thermoelectric semi-conductor devices operate on the Peltier effect and produce either heating or cooling depending on the direction of flow of electric current therethrough and the thermoelectric devices are interposed between a heat conducting plate at one side that is disposed in heat conducting relation with the bubbler and a heat sink on the other side. When they are operated to cool the plate, it is necessary to dissipate heat from the heat sink and conversely, when the thermoelectric devices are operated to heat the plate, it is necessary to warm the heat sink. Fans and blowers have been used to pass air over the heat sink to either heat or cool the sink, depending on the mode of operation of the thermoelectric device. The thermoelectric devices use relatively high operating currents and it is also desirable to cool the circuitry for operating thermoelectric devices to prevent overheating of the circuit components and to minimize temperature drift in the circuitry.
In some prior art thermoelectric temperature controllers for bubbler containers, the power supply circuitry for the thermoelectric device was mounted in a separate housing that could be located remote from the housing that contained the thermoelectric device and which received the bubbler container. In such an arrangement, a single fan could not be used to both pass air over the heat sink and to circulate air past the circuit components for cooling the same. In a prior art thermoelectric cooling apparatus manufactured and sold by the assignee of the present application, a single housing was used and separated by an upright partition into a compartment for receiving the bubbler container at one side of the partition and a compartment for receiving the circuit components at the other side of the partition, and a thermoelectric heating and cooling unit below the container receiving compartment. In this prior art temperature control apparatus, a squirrel cage type blower was mounted in the components receiving compartment and was arranged to draw its intake air in through the components compartment and discharge the air over a heat sink of the thermoelectric cooling device. This arrangement was found to be unsatisfactory particularly when the thermoelectric device was operated in a mode to cool the bubbler, since the intake air to the blower was preheated by the circuitry in the components compartment and produced insufficient cooling of the heat sink for the thermoelectric device under some operating conditions. It was also found that re-arranging the air flow to a fan in the components compartment so that air was drawn into the fan over the heat sink of the thermoelectric device and then discharged through the components compartment, did not provide satisfactory operation, particularly when the thermoelectric device was operated in its cooling mode. The intake air to the fan was preheated by the heat sink of the temperature control device and did not provide adequate cooling of the components in the component compartment so that temperature drift and overheating of some components occurred under the adverse temperature conditions in the gas cabinet.